The mechanical properties of a composite carbon-resin material improve with an increase in the shear stress at which interlaminer decohesion occurs, and consequently with improved adherence between the carbon fibers and the resin. However, very high adherence gives rise to a degree of fragility in the material, i.e. a toughness defect.
Proposals have already been made to improve the adherence of fibers to resin by applying surface treatment to raw carbon fibers as manufactured, either by chemical means or else by electrochemical means. Chemical groups are thus produced on the surface of the fibers to improve fiber adherence to resin, to a large extent by creating chemical bonds between the fiber and the matrix, but also to some extent by increasing the Van der Waals interactions or the bipolar interactions between the two fiber and resin components, where applicable.
Electrochemical treatments of this type are described, for example, in published French patent application No. 2 477 593. They consist essentially in immersing the fibers in an electrolyte solution and in polarizing the fibers positively relative to a cathode. Good adherence is obtained, in particular, by using as electrolytes sulfates and bisulfates of ammonium and sodium which are strong salt electrolytes.
These electrolytes include oxygenous anions and cause oxygenous groups to be grafted onto the carbon fibers. These oxygenous groups improve fiber adherence with synthetic resins, but the method of treatment can sometimes degrade the mechanical properties of the carbon fibers.
The above-mentioned prior application also refers to treatment performed using strong bases and strong acids as electrolytes (sulfuric acid, phosphoric acid, sodium hydroxide). It is then observed either that the hardening of impregnated resin is inhibited, or else that the treated fibers have poor resistance to thermal oxidation.
An examination of the operating conditions of conventional electrochemical treatments shows that:
in general, they use acid, basic, or salt solutions in an aqueous medium; and
the potential applied between the anode constituted by the carbon fibers and the cathode is great enough to decompose water causing gaseous oxygen to be evolved, a well-known electrochemical phenomena.
The electrolyte then includes reactive species which attack the carbon of the fibers to form oxygenous surface groups that promote fiber-matrix adhesion. The potential V.sub.0 at which water decomposes and evolves oxygen is about +1.7 volts relative to a saturated calomel reference electrode, but it may be less in some electrolytes. In any event, anode treatments performed at more than V.sub.0 always give rise, regardless of the electrolytes used, to water decomposition and to the formation of oxygenous groups (of the C.dbd.O, COH, COOH, . . . type), and even to a degradation of the surface of the fibers if the working potential V.sub.t is much greater than V.sub.0. Only the relative proportions and surface concentrations of the oxygenous groups vary from one method to another, and one can hardly expect an improvement in the toughness of the resulting composite materials since the fiber-matrix interface provided by the oxygenous groups is of essentially the same nature from one treatment to another.
The Applicant's published French patent application No. 2 564 489 describes a method of surface treating carbon fibers in order to graft nitrogenous functions thereon. In this method, the fibers are immersed in an aqueous solution of an amine compound that dissociates water little, so as to avoid lowering V.sub.0 too much.
The aim of the invention is to provide an electrochemical method causing nitrogenous groups to be grafted onto the surface of carbon fibers, while avoiding the limitations related to the use of an aqueous solution, in particular with respect to the speed of the electrochemical reaction.
Another aim of the invention is to graft nitrogenous groups onto carbon in a form other than carbon fibers, in particular in divided form, for example for use as a catalyst.